The present invention relates to the field of semiconductor wafer testing and, more particularly, to the measurement of the electronic conduction behavior of various insulating layers.
The production of insulating layers, particularly, thin oxide layers, is basic to the fabrication of integrated circuit devices on semiconductor wafers. A variety of insulating dielectric layers are used for a wide range of applications. These insulating layers can be used, for example, to separate gate layers from underlying silicon gate regions, as storage capacitors in DRAM circuits, for electrical device isolation and to electrically isolate multilayer metal layers. The electrical insulating properties of these layers is of great interest. Some of the measures of the electrical insulating quality of these layers are (1) the conduction current at a given applied voltage or applied electric field strength, (2) the voltage or electric field strength corresponding to a given applied conduction current and (3) the terminal value of a saturating increase in voltage or electric field strength (tunneling field) corresponding to a regime of rapidly increasing conduction with increasing voltage or field.
Another property of interest, related to the insulating properties of an insulating layer is the electrical breakdown voltage of the dielectric layer. Voltage can be increased across an insulating layer until a sudden increase in conduction current is observed. If this current is due to a localized fault or a physical pinhole in the dielectric, then it is often referred to as a defect breakdown. If the current is due to a field induced impact ionization mechanism that takes place rather uniformly over the entire cross section of the dielectric, then the field produced by the applied voltage (i.e., voltage over thickness of the dielectric) is considered to be the intrinsic breakdown field of the material.
It is noted that for thermal oxides (e.g., silicon dioxide), for example, the tunneling currents can be so high as to tend to obscure the intrinsic breakdown characteristic of the material.
Various methods involving the use of contacts at the oxide surface have been used for measurement of current-voltage behavior. These methods suffer from sensitivity to pinholes in the oxide and from localized breakdown effects associated with the contacting electrodes.
A typical approach has been to deposit a metal layer on top of the oxide and to then apply a test voltage to the oxide. This not only affects the measurement, but also will typically spoil the test wafer.
The current-voltage behavior of an insulator can be influenced by the composition of the layer as well as the thermodynamic growth conditions of the layer. Other perturbing influences can be electron trapping states in the layer, polarity dependent carrier injection from the silicon and the particular type of deposited electrode that is used to perform the measurements. Quite often, particularly for thin oxides, the ability to make measurements is thwarted by pinholes in the layer that become localized highly conductive paths after a measurement electrode is deposited. The present invention is advantageously insensitive to localized faults and pinholes.
U.S. Pat. No. 5,498,974 is incorporated herein in its entirety by reference. The patent discloses an apparatus for depositing corona charge on an insulating layer and for measuring the voltage on the surface of the layer.